This invention relates to a solenoid pilot operated change-over valve employing a spool/piston assembly which is movable in response to pressure in a pilot chamber for switching flow of input pressurized fluid from one to another output port of the valve.
The conventional solenoid pilot operated change-over valves of this sort are generally constructed as shown in FIG. 1. The valve of FIG. 1 has a spool A in a first change-over position connecting an input port B with a first output port C. Upon energization of a solenoid which is mounted around a pilot valve chamber J on the outer side of the piston F, a plunger E in the pilot valve chamber is moved away from the piston F to uncover a pilot valve seat G on the piston F and attracted on the core of the solenoid to close a drain valve seat H which is provided in the core. As a result, a pilot pressure is built up in the pilot chamber J due to the fluid pressure which is supplied from the input port B through an axial passage I in the spool A, moving the piston F to the inner end of a piston chamber K. Simultaneously with the movement of the piston F, the spool A is shifted to a second change-over position connecting the input port B with a second output port L.
Under these circumstances, if the solenoid D is de-energized, the plunger E is moved away from the drain valve seat H by the action of a spring M to return to its initial position, opening the drain valve seat H while closing the pilot valve seat G on the piston F which is now at the inner end of the piston chamber K. As a result, the fluid pressure in the pilot chamber J is released through the drain valve seat H, whereupon the spool A and piston F are returned to the first change-over position by a return spring N which is mounted in the spool chamber to act on the other end of the spool A, bringing the plunger E to the initial position shown in FIG. 1.
One of the drawbacks inherent to the conventional change-over valve as shown in FIG. 1 is that it is necessary to drive the plunger through a great distance when shifting the spool/piston assembly from the second to the first change-over position since the piston is in the remotest position from the plunger. To attain this, one has to load a spring of large power and stroke length on the plunger, and at the same time mount a solenoid of large power which can cope with the large plunger-driving spring. For these reasons, it has been difficult to provide a solenoid pilot operated change-over valve which is compact and small in size.
It is an object of the present invention to provide a solenoid pilot operated change-over valve of the type mentioned above in which the spool/piston assembly is automatically reset in the initial position upon de-energization of the solenoid before the pilot valve seat is completely closed by the plunger. More particularly, the invention provides a solenoid pilot operated change-over valve wherein the effective diameter of the drain valve seat in the pilot chamber is formed at least 1.4 times greater than that of the pilot valve seat on the piston through which the pilot chamber is communicable with the input port of the valve.
It is another object of the present invention to provide a solenoid pilot operated change-over valve which employs a plunger of a minimized stroke length.
It is still another object of the invention to provide a solenoid pilot operated change-over valve which is compact and simple in construction.